This has been a pivotal week of campaigning for Barack Obama. It is too early to say he has broken through in the race for the Democratic nomination. There are important contests to come in Ohio, Texas and Pennsylvania, where he is now running far behind Hillary Clinton in the polls. He has more delegate votes that Mrs Clinton, but not enough yet to make the outcome a foregone conclusion. But after winning eight successive primaries and caucuses since Super Tuesday, some by large margins, Mr Obama is obviously the man to beat. The Republican frontrunner, John McCain, confirmed it this week by attacking him directly for the first time. The Obama campaign looks confident: this week he shifted from highlighting the movement he has created to detailing the substance of its message. He is addressing what is turning out to be a central issue in the election, the economy, and reaching out to all groups, not least white working-class voters.

What does this tell us about qualities of the man? First, that he is tough. No one is calling him Obambi any more. Looked at as a business, his election campaign is a multimillion-dollar concern that employs around 500 people, who show loyalty and discipline. He runs a tight ship, which is more than can be said for his Democratic opponent. Hillary Clinton has made much of her 35-year experience of government, but her campaign has suffered from sharp mood swings, conflicting messages, forced departures and defections - the latest being David Wilhelm, who managed her husband's 1992 campaign for the White House.

Second, Mr Obama is a man who sticks to his original course and has the energy and determination to make it prevail. If he wins, his campaign will be studied as a textbook example of the art. His slogan "Yes We Can" may make outsiders cringe, but the optimism of it is part and parcel of the American Dream. You can wrap a Stars and Stripes around it. From now until the Democratic convention in Denver in August, the contest will be between a candidate, in Mrs Clinton, fighting tooth and claw for the party's traditional base, and a candidate, in Mr Obama, who creates new constituencies, energises people who have never voted before and appeals to moderate Republicans as well as traditional Democrats. The idea that a new generation is coming to power ("we are the change we have been waiting for") and that the tectonic plates are shifting beneath the surface of American politics is a theme on which only one of three viable candidates left in the race for the presidency has been able to capitalise.

There is no reason to assume that Mr Obama cannot deliver as president. He already has delivered as a candidate. Yet what he will achieve in power will inevitably fall short of expectation. Take Iraq, for example. The situation in the country is changing quickly, which may make it easier for him to honour his commitment to pull out troops. But if his opposition to the war has made him popular in Europe, it will still be a handicap against Mr McCain - a war veteran, a hero and a man who has no difficulty making the case that he would be the better commander in chief.

And yet, barring another major attack by al-Qaida, America is growing weary of the Orwellian message of a war without end. That is the story of the current administration, which will go down as one of the worst in America's history. The next administration must be different. This is the ground on which Mr Obama aims to fight: against recession, property foreclosures, job losses and higher prices. Everyone should be grateful that neither Democratic candidate has so far indulged in protectionist talk, or sought to isolate America from the world outside. For all the tension and name-calling, the contest so far has been gripping, even uplifting. Mr Obama deserves much of the credit for this optimistic mood. It could - and should - carry him to the White House.